testsuite: Require bitint in gcc.dg/pr126538.c

The new gcc.dg/pr126538.c FAILs on SPARC:

FAIL: gcc.dg/pr126538.c (test for excess errors)

Excess errors:
gcc.dg/pr126538.c:4:9: sorry, unimplemented: '_BitInt(65535)' is not supported on this target

Fixed by requiring bitint.

Tested on sparc-sun-solaris2.11 and i386-pc-solaris2.11.

2026-08-26  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	* gcc.dg/pr126538.c: Require bitint.